Thermoelectric converters or devices enable “cold” (refrigeration/air conditioning applications) to be produced, but it is also an effective way to recover waste heat to produce electricity. In this latter field, they are considered for use in the automobile industry, for example, for conversion of the heat from the exhaust gases.
Examples of other fields of application of the thermoelectric elements and converters as presented hereinafter are fields of application such as micro-cogeneration particularly from biomass, supplying electricity to Internet connected objects, power supply for networks of sensors, power supply for systems in remote and hostile environments, as well as power supply for the space domain.
It is known that a reduction of the dimensions of thermoelectric elements, also called thermoelectric legs, makes it possible to obtain an increased electric power density. Reference can be made to the documents U.S. Pat. No. 6,388,185 B1 and U.S. Pat. No. 5,712,448 A1. Similarly, it is known that a temperature increase at the terminals of the thermoelectric legs makes it possible to increase the electric power density. However, because of the significant thermomechanical stresses related to differential thermal expansion coefficients between its various constituent elements; it is too often considered very difficult or even impossible to reduce the dimensions of such thermoelectric elements while still ensuring reliable operation with an appreciable temperature difference.
Indeed, thermoelectric materials in bulk form generally have a fragile mechanical behavior, and thermal mechanical stresses cause irreversible damage (cracks), particularly near the interface with the metal electrodes or at the center of the thermoelectric material when the material is of good quality. This results in significantly breaking-down the performance of the thermoelectric converter.
Added to this difficulty is that related to problems of chemical diffusion between the thermoelectric legs and the electrodes, problems that intensify with increases in temperature. The diffusion phenomenon leads to a progressive break-down of the thermoelectric and mechanical properties, which ultimately lead to a malfunction of the device.
The most successful thermoelectric generator incorporating thin legs was the one developed by the MICROPELT Company. Reference can be made, for example, to the document U.S. Pat. No. 7,402,910 B2. However, the used thermoelectric material, based on Bi2Te3, and the method of manufacturing thermoelectric elements by physical deposition (thin layers), establish severe practical limits on the useful temperature range (around the ambient temperature) and on the dimensions of the converters (several tens of microns thick). This thin layer-based technology therefore remains restricted to applications where the needs for electrical power do not exceed some 10 mW.
To overcome the technical problems set forth above, present day thermoelectric generators are composed of n- and p-type elements having a length on the order of one centimeter. These elements are brazed to metal electrodes with a thin layer of brazing material.